The present invention relates to a burner for a socalled "powdery coal" or "powdered coal" which is provided by finely pulverizing coal (hereunder referred to as "powdery coal burner").
Recently, since oil prices have been sharply increasing, coal fuel has been re-emerging as a source of energy in place of oil fuel.
In order to increase the efficiency of the combustion of coal, the coal should be burned in a powdery form, i.e. powdered form. However, the usage of powdery coal is generally limited to large capacity burners such as found in power plant boilers, cement-manufacturing kilns and the like. This is due to fact that the combustion rate of coal is lower than that of gaseous or liquid fuels, and that the flame characteristics of coal are quite different from those of the gaseous or liquid fuels. Thus, powdery coal has almost never been used in small capacity burners for the following reasons:
(i) The total amount of heat accumulated in the combustion field of small capacity burners is much smaller than that of large capacity burners, making it impossible to maintain a continuous combustion of coal in small capacity burners even when utilizing powdery coal. This is because of the fact that powdery coal is characterized by a higher catch-fire point and less flame stability in comparison with gaseous or liquid fuels.
(ii) The combustion rate of powdery coal is very slow, making it difficult to produce sufficient combustion within a limited area. In addition, a combustion flame is sometimes formed in an area remote from the front end of the burner. Therefore, the formation of a large amount of partly combusted coal particles and other fuels (hereunder referred to as "uncombusted matters") is inevitable in small capacity burners, which essentially require not only the achievement of a high temperature, intensive combustion within a limited area, but also the formation of a short flame.
Several methods have been known to increase the combustion rate of fuels. One of them is applying a swirling motion to the combustion air and/or the fuel itself. When such a conventional method is applied to powdery coal burners, though it is possible to shorten the flame length, the coal powder to which the swirling motion has been applied to flows in radial directions which excessively extends the combusion flame area, sometimes resulting in precipitation of fused coal onto a wall area ajacent to the burner nozzle under high temperature conditions. This is called "clinkering" and sometimes obstructs the operation of the burner. If the furnace temperature is relatively low, coarse coal particles will be blown from a high temperature area around the axis of the burner to a low temperature area in the periphery of the combustion flame, resulting in a relatively large amount of uncombusted matters.